(1) Background: To investigate the real-world effectiveness and safety profile of nusinersen in Croatian paediatric and adult spinal muscular atrophy (SMA) patients. (2) Methods: A retrospective and anonymous collection of relevant demographic and clinical data for all Croatian SMA patients treated with nusinersen and reimbursed by the Croatian Health Insurance Fund (CHIF) between April 2018 and February 2022 was performed through searching the CHIF database and studying the associated reimbursement documentation. All patients who received at least one dose of nusinersen were included in the baseline clinical-demographic overview and safety analysis, whereas only subjects who had completed six doses were included in the effectiveness analysis. (3) Results: Fifty-two patients [61.5% male; median age 13.4 (0.1–51.1) yr.] received nusinersen treatment. In SMA type 1 and type 3 paediatric patients, statistically significant motor function improvement (CHOP INTEND 10.8 ± 10.3 vs. 20.0 ± 15.8, p = 0.003; HFMSE 49.6 ± 7.9 vs. 53.1 ± 7.7, p = 0.008; respectively) was achieved immediately after 4 loading doses of nusinersen and remained statistically significant onwards. Average improvements in HFMSE motor performance in SMA type 2 patients after four, five, and six doses of nusinersen were +6.0, +10.5, and +11.0 points, respectively. In SMA type 3 adult patients, no significant improvement in RHS motor performance or the 6-Minute Walk Test (MWT) was demonstrated. During the study period, 437 doses were administered without any new safety concerns appearing. (4) Conclusions: Our RWD findings indicate that nusinersen is an effective and safe treatment in a heterogeneous group of paediatric patients with all types of SMA; however, no significant benefit (but only RHS and 6MWT maintenance) was demonstrated in SMA type 3 patients who started nusinersen after >18 years of age.